Bygone London transport (4)
I believe the answer is:
tram
'bygone london transport' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tram that I've seen before include "Urban transport" , "Vehicle running on rails in road" , "It runs on rails" , "Road vehicle running on tracks" , "The Luas, say" .)